The central role of sodium in the active transport of sugars and amino acids has been demonstrated by numerous techniques in a wide variety of tissues. However, little is known about the molecular components and mechanisms underlying Na-dependent transport. It is clear that only after this type of information is available will one be able to construct a fundamental understanding of these important processes. The primary aim of the research proposed is to obtain molecular information about Na-coupled neutral amino acid transport across the mucosal border of rabbit intestine, a well-characterized Na- dependent system ideally suited for molecular studies. Information as to the types and locations of macromolecules and their constituent chemical groups involved in this transport system will be sought through the study of physical and chemical modification (both general and specific) of Na-coupled neutral amino acid influx and efflux at the mucosal (brush) border and the isolation and study of Na-coupled amino acid transport in mucosal villous cells (cells with brush borders) and crypt cells (cells without brush borders).